Sixteen factors and three subfactors related to RTW-ES were identified after analyzing arguments and grounds of LE's derived from a case with an employee sick-listed due to depressive disorder. Of these 16 factors, 9 were fitted within the domains of the ICF model: 'functional capacity' and 'job demands vs. functional capacity' (activity domain), 'competencies', 'attitude', 'illness perception', and 'self-efficacy' (personal factors); 'work-relatedness of sickness absence', 'job availability', 'employer size', 'employer attitude', 'relationship between the employer and employee', 'communication quality and quantity' (environmental factors). Seven factors did not fit within the ICF model because they describe actions undertaken during the RTW process. These factors are categorized as interventions ('training/education', 'job offerings', 'professional advice'), job accommodations ('temporary/modified duty', 'change of employer'), and measures ('research/assessment', 'monitoring').
A comparison of the factors relevant in the assessment of RTW-ES in employees sick-listed due to a DD and due to CLBP [4
] shows that 16 factors and 2 subfactors are relevant to both health conditions. The three factors which were only considered relevant in cases of CLBP are 'age', 'educational level' and 'tenure'. One sub-factor, 'suitability of own work', was considered relevant in the cases of CLBP only, and another sub-factor, 'reorganization', was considered to be relevant only in the case of DD.
When RTW processes in physical and mental health conditions are compared, initial RTW interventions mainly focus on reducing the symptoms of the health conditions [13
]. In physical health conditions the focus is on reducing the symptoms in physical functioning, in mental health conditions the focus is on reduction of stress, coaching and supervisor support [14
]. However, in long-term sickness absence, RTW trajectories in both health conditions become more similar because psychosocial factors rather than symptoms influence the RTW process. When long-term conditions are considered, disease specificity is significantly reduced [16
], which is consistent with our comparison of DD and CLBP. Our findings of similarity between the factors relevant to the assessment of RTW-ES seem to be in line with the literature on factors relevant in the RTW process of employees on long-term sickness absence. However, these differences between factors relevant in cases of DD and CLBP could be attributed to case differences. For example, the difference in relevance of the factor 'age' might be contributed to differences in the cases used. In the cases of CLBP, the employees in question were older than in the case regarding DD (50 and 57 vs. 47 years old). The chance of RTW decreases when employees are over 45 years of age [19
]. The participating LE's in our study explained in the CLBP cases that they expect the employer to undertake more efforts when the age of the employee is increased [4
]. In the case of DD it could be that LE's do not consider an age of 47 a relevant factor for RTW-ES. According to literature, age is important to RTW in both mental and physical health conditions [13
]. However, the relation between age and time to RTW is less pronounced in mental health conditions. In a study of Koopmans et al. (2008) it was found that the factor 'age' is less relevant in mental health conditions, because time to RTW is already high in younger age groups [17
]. However, some factors could be considered disease-specific. It could be that for example reorganization is only relevant in DD because reorganizations could cause a more stressful environment, thereby decreasing the chances of RTW in DD, but this might not have an added effect in CLBP. Future research is necessary to investigate the influence of case differences and disease specificity in the assessment of RTW-ES.
The relevance of this study lies in its unique topic of interest. The assessment of RTW-ES is an important part of the RTW process, but little information and no evidence-based guidelines are available. Investigating factors relevant in the assessment of RTW-ES is crucial for the optimization of the quality of the RTW process. Knowing which factors are relevant in the assessment of RTW-ES by means of research and including this kind of information in the existing protocol will optimize not only the transparency and reliability but also the validity of the assessment. Based on these results, a multifactorial approach to the assessment of RTW-ES is essential. A guideline focusing on all relevant factors could improve the quality of the assessment of RTW-ES and could support professionals involved in the RTW process in designing and monitoring the RTW process and the activities undertaken during this process. This optimized assessment should therefore not only benefit the LE's assessing RTW-ES, but also other stakeholders involved in the RTW process, e.g. employer, employee and health professionals. Considering the importance of the assessment of RTW-ES, this optimization is essential.
The main strength of this study lies in its investigation of this unknown territory by means of focus group research. Focus groups have proven to be an effective method to gather information about implicit knowledge of professionals [11
]. When literature is lacking, expert knowledge is considered an appropriate means of collecting evidence. Also, we have used two focus groups, to ensure a minimal amount and diversity of response. The two focus groups yielded a different number of arguments, however, both groups touched the same major issues when discussing the arguments and underlying grounds.
Moreover, during the assessments, additional information was presented by actors in order to reflect a more realistic situation. We agree that this method of presenting additional information could be a source of variation. However, we have chosen for this method because it reflects a more realistic situation, and we wanted the assessment to be as close to the actual situation as possible.
A possible limitation of this study is the use of only one case. Also, the case depicted a specific case of DD, while there are many appearances of DD. However, the effect of the use of this single case is unknown. While the use of more cases or different cases might have yielded different results, the majority of the results which have been found in relation to the cases about CLBP have been replicated in this study.
Future research should focus on the reproducibility of this study in a different context by investigating different cases, with different health conditions, DD appearances, or employee characteristics (e.g. gender). Using different focus groups and legislatory context could improve the generalizability of the results and provide more insight into the assessment of RTW-ES. Also of interest for further research would be the effect of the use of the factors on the reliability of the assessment. Providing a list of the relevant factors could increase the homogeneity and thereby the quality of the assessment of RTW-ES.